Since everyone has given good suggestions about layout, i would say that depending on your design domain :
For Digital
Now-a-days digital boards among which FPGA, microcontrollers, Arm Processor based boards will be the application interests, designing with microcontrollers would be relatively easy but whereas for designing with FPGAs would entail some effort, some concepts are of critical interest like Configuration, Interfaces, Power, Banks etc., and for these i would suggest you to go to Xilinx website and look into user guides of any FPGA family and also look into the schematics of the evaluation platforms that they provide (you can download them).
You can also look at Beagleboard schematics they are open source with really nice
user guide which describes each and every circuit on board.
For Analog -- I don't know what to suggest, all i know is keep it away from digital section of your board for grounding reasons.
Power
Power Distribution Network design is the most critical aspect of any board if this aspect is not efficiently addressed then don't expect the rest of the board to function, learn to do some power budget calculations of your board by looking into the datasheets of all your ICs and calculating their worst case maximum current needs etc.
I Hope this helps.